U.S. Pat. No. 4,194,505 discloses an injection device of this type in which the dosage can be selected by the user. This known device is designed to accomodate a special cartridge or ampule. The ampule contains for example a certain amount of insulin, for example 20 units. If the patient needs only 17 units, he can adjust the injection device correspondingly. First, the patient adjusts the injection device for the maximum dosage; to do this, the patient uses a null- or zero-marking on the injection device. Next, the patient adjusts an adjuster for three lines on a scale; this corresponds to the 3 units that the patient does not require (20-27 =3). Then he performs the injection.
This known injection device has two considerable disadvantages:
a) Only adjustment within a very limited range is possible. For example, if the patient requires at one time 28 units of insulin, he can make use of a 30-unit ampule, because the difference of two units is well within the range of adjustability. However, if another time the patient requires 19 units, it is no longer possible to use a 30-unit ampule, and a 20-unit ampule must be used. Therefore, it is necessary to use ampules that contain different quantities, namely different ampules that contain 10, 20, 30, 40, 50 or 60 units. If the patient is blind, he can easily make a mistake and use the wrong ampule. There is then the danger that he will inject too much insulin.
b) The computation and adjustment work involved in utilizing, e.g., a 20-unit ampule to perform a 17-unit injection, is easy for many persons. However, for many other persons this computation and adjustment work is too complicated. For example, with so-called "intensivated insulin therapy", the patient must inject a first amount of insulin after breakfast, a different second amount after lunch, and a different third amount after dinner. An example: The patient needs e.g. 8 units after breakfast, 15 units after lunch, and 12 units after dinner. Therefore, for breakfast the patient must use an ampule containing 10 units and adjust the device to -2 units. For after lunch and for after dinner, he must use an ampule having 20 units. However, for after lunch he must adjust the injection device to -5 units, and for after dinner he must adjust the device to -8 units. This computation and adjustment work requires a certain degree of care and can easily lead to mistakes.